Mix chicken, green beans, corn, water chestnuts, celery soup, mushroom soup, chopped onion, sour cream, and rice. Coat a 9×13 casserole dish with cooking spray. Pour half of mixture into dish and top with half of shredded cheese. Add remaining mixture and top with rest of cheese. Melt butter and mix well with Ritz crackers, sprinkle over top of casserole. Bake at 350 degrees for 30-35 minutes.